This invention relates in general to a process for removing adherent material, such as paint, scale or other coverings from hard metal surfaces and, more particularly, to a non-chemical surface cleaning process employing mechanical blast.
For various types of structures, it is often necessary or desirable to remove any layer of coating which has been previously applied to or formed on surface areas. Numerous techniques exits for removing paint, sealants, lacquers, rust, scale and other adherent materials from virtually any type of surface. Surface cleaning or stripping methods range from mechanical abrasion to the use of strong chemicals and involve varying degrees of time, effort and expense. For any given type of coating, the character and function of the substrate material from which a coating is to be removed usually dictates the stripping method, at least in industrial settings. Hard, durable surfaces, such as heavy steel plating can be cleaned or stripped by relatively fast abrasive methods such as sand blasting. Softer metals such as aluminum or more delicate surfaces such as polymer composite layers may require the use of a softer abrasive material during blasting or require careful chemical removal to prevent damage or destruction of the substrate.
Sand blasting of steel plate to remove adherent coatings and the like, while successful in removing the coatings, has several disadvantages. For one, the sand abrasive is very friable such that upon contact with the surface, a vast amount of silica dust is formed. Recently there has been a concern that the minute silicious particles which are formed present a substantial health hazard, in particular, if ingested into the lungs. Secondly, very large amounts of sand are required for cleaning large structures such as bridges, stacks, etc. such that after blasting, this sand remains and must be removed from the area adding substantially to the time and expense of the process.
Alternative abrasives are known. For example, U.S. Pat. No. 3,775,180 is directed to a method for descaling steel in which the steel is descaled by spraying a mixture of a solid such as aluminum oxide or silicon carbide with water and a gas such as air under specified conditions onto the steel. In removing a coating or a scale on the surface of a metal, however, it is important that the anchor pattern (surface roughness) of the metal surface be uniform and not to extensive such that the surface and even the metal structure is damaged. A blast media composed only of hard aluminum oxide and silicon carbide can be detrimental to the metal structure. For certain surfaces such as metals softer than steel, a softer abrasive can be used with the blast stripping method. An example of such is disclosed in U.S. Pat. No. 4,878,320 to remove coatings from aluminum, fiber glass or carbon fiber laminate. As disclosed in the patent, an abrasive particle is used which has a Mohs hardness of about 3. Sodium bicarbonate is the preferred material. Likewise, the present assignee markets a sodium bicarbonate blast media such as for removing paint, scale and the like from aluminum under the tradename Armex. Although very effective for removing coatings from aluminum or other softer materials such as polymer composites and the like, sodium bicarbonate abrasive is not hard enough to provide a sufficient anchor pattern on hard surfaces such as steel so as to prime the surface for the addition of a new coating layer. A large advantage of sodium bicarbonate as a blast media is that the material is water soluble and non-toxic so that a blasting area can be cleaned relatively easily by washing the media away with water without a harmful environmental effect.
Other patents which disclose cleaning metal surfaces with an abradant other than sand includes U.S. Pat. No. 2,624,988 which utilizes Tripoli paste and a liquid vehicle and which can be mixed with sponge rubber fragments which carry the abradant to the metal surface and which provide a rubbing action to polish and buff the metal surface. The addition of a small amount of alumina to the Tripoli paste is disclosed under certain conditions.
U.S. Pat. No. 2,710,286 discloses a method of removing fluorescent and other materials from viewing screens of cathode ray tubes in which sodium and potassium carbonate are used as the abrasive material. U.S. Pat. No. 4,588,444 discloses removing calcium from polymeric contact lenses by using as an abradant sodium chloride, sodium bicarbonate or a mixture of same. U.S. Pat. No. 4,731,125 discloses a method for removing adherent material from composite surfaces made of a reinforced matrix material using a granular media composed of particles which have a Mohs hardness of lower than 3.5. Preferably the abradant is polymeric particles having the desired Mohs hardness.